Shortening the rail is a non-starter. First off, this is a customer's locomotive, so fixing it on my layout won't do him any good. FWIW, I'm planning on tweaking all of the rails on this layout in any case as I have a couple of problem engines that hit the on the side. However, as wide as these wheels are, I don't think just shortening the rails will do it unless I really went crazy. However, since it doesn't really have to run on my layout, I'd have to fly out there and shorten all of his rails to actually fix the issue. 
I have no idea where I'd find compatible plastic wheels, especially the color of these. Also, I'm not really willing to go on a snipe hunt in hopes of finding something that would work and look right.
Given the nature of these wheels as well as the size, I think my only option is to modify this set of wheels. In my mind, the tapering is really the best choice, but I only get one shot at it. If I screw it up, I have a bigger issue in that I have to find the proper wheels to fix my screwup. So, my mission is not to screw up! 
